Design and characterization of a soluble fragment of the extracellular ligand-binding domain of the peptide hormone receptor guanylyl cyclase-C

The intestinal guanylyl cyclase-C (GC-C) was originally identified as an Escherichia coli heat-stable enterotoxin (STa) receptor. STa stimulates GC-C to much higher activity than the endogenous ligands guanylin and uroguanylin, causing severe diarrhea. To investigate the interactions of the endogenous and bacterial ligands with GC-C, we designed and characterized a soluble and properly folded fragment of the extracellular ligand-binding domain of GC-C. The membrane-bound guanylyl cyclases exhibit a single transmembrane spanning helix and a globularly folded extracellular ligand-binding domain that comprises about 410 of 1050 residues. Based on the crystal structure of the dimerized-binding domain of the guanylyl cyclase-coupled atrial natriuretic peptide receptor and a secondary structure-guided sequence alignment, we generated a model of the extracellular domain of GC-C comprised of two subdomains. Mapping of mutational and cross-link data onto this structural model restricts the ligand-binding region to the membrane proximal subdomain. We thus designed miniGC-C, a 197 amino acid fragment that mimics the ligand-binding membrane proximal subdomain. Cloning, expression and spectroscopic studies reveal miniGC-C to be a soluble and properly folded protein with a distinct secondary and tertiary structure. MiniGC-C binds STa with nanomolar affinity.

hK5 and hK7, two serine proteinases abundant in human skin, are inhibited by LEKTI domain 6

BACKGROUND

Several skin diseases and atopic disorders including Netherton syndrome and atopic dermatitis have been associated with mutations and deviations of expression of SPINK5, the gene encoding the human 15-domain serine proteinase inhibitor LEKTI. The biochemical mechanisms underlying this phenomenon have not yet been fully clarified.

OBJECTIVES

To identify target proteinases of LEKTI important for processes of desquamation and inflammation of the skin which will enable the development of specific drugs.

METHODS

The inhibitory activities of LEKTI domains 6 and 15 were tested on a number of commercially available serine proteinases and also on the purified kallikreins hK5 and hK7. In addition, recombinant hK5 was used.

RESULTS

LEKTI domain 6 is a potent inhibitor of hK5 and hK7, whereas LEKTI domain 15 exhibits inhibitory activity on plasmin. hK5 and hK7 in particular are relevant to skin disorders.

CONCLUSIONS

The inhibition of hK5 and hK7 by LEKTI domain 6 indicates an important regulatory role of LEKTI in processes of skin desquamation and inflammation, which may explain the severe pathological symptoms associated with abnormalities of SPINK5 and/or its expression. Thus, LEKTI represents a potential drug for the treatment of these disorders.

Accurate disulfide formation in Escherichia coli: overexpression and characterization of the first domain (HF6478) of the multiple Kazal-type inhibitor LEKTI

The human hemofiltrate peptide HF6478, a putative serine proteinase inhibitor, which is part of the precursor protein LEKTI, was cloned, overexpressed, and purified. HF6478 contains two disulfide bridges with 1-4, 2-3 connectivity, sharing partial homology to Kazal-type domains and other serine proteinase inhibitors. It was expressed as thioredoxin (Trx) fusion protein, and disulfide formation occurred in the oxidative cytoplasm of Escherichia coli Origami (DE3) strain which carries a trxB(-)/gor522(-) double mutation. The soluble fusion protein was purified using metal-chelating affinity chromatography. Cleavage of the Trx fusion protein with factor Xa and subsequent purification yielded the final product in amounts sufficient for structural studies. Characterization of recombinant HF6478 was done by amino acid sequencing, mass spectrometry, capillary zone electrophoresis, and CD spectroscopy. Taking the blood filtrate peptide HF6478 as example, we present a strategy which should facilitate the expression of different extracellular proteins in the E. coli cytoplasm.

Receptor recognition by a hepatitis B virus reveals a novel mode of high affinity virus-receptor interaction

The duck hepatitis B virus model system was used to elucidate the characteristics of receptor (carboxypeptidase D, gp180) interaction with polypeptides representing the receptor binding site in the preS part of the large viral surface protein. We demonstrate the pivotal role of carboxypeptidase D for virus entry and show its C-domain represents the virus attachment site, which binds preS with extraordinary affinity. Combining results from surface plasmon resonance spectroscopy and two-dimensional NMR analysis we resolved the contribution of preS sequence elements to complex stability and show that receptor binding potentially occurs in two steps. Initially, a short alpha-helix in the C-terminus of the receptor binding domain facilitates formation of a primary complex. This complex is stabilized sequentially, involving approximately 60 most randomly structured amino acids preceding the helix. Thus, hepadnaviruses exhibit a novel mechanism of high affinity receptor interaction by conserving the potential to adapt structure during binding rather than to preserve it per se. We propose that this process represents an alternative strategy to escape immune surveillance and the evolutionary pressure inherent in the compact hepadnaviral genome organization.

Solution structures of human parathyroid hormone fragments hPTH(1-34) and hPTH(1-39) and bovine parathyroid hormone fragment bPTH(1-37)

Parathyroid hormone (PTH) is involved in regulation of the calcium level in blood and has an influence on bone metabolism, thus playing a role in osteoporosis therapy. In this study, the structures of the human PTH fragments (1-34) and (1-39) as well as bovine PTH(1-37) in aqueous buffer solution under near physiological conditions were determined using two-dimensional nuclear magnetic resonance spectroscopy. The overall structure of the first 34 amino acids of these three peptides is virtually identical, exhibiting a short NH(2)-terminal and a longer COOH-terminal helix as well as a defined loop region from His14 to Ser17, stabilized by hydrophobic interactions. bPTH(1-37), which has a higher biological activity, shows a better-defined NH(2)-terminal part. In contrast to NH(2)-terminal truncations, which cause destabilization of helical structure, neither COOH-terminal truncation nor elongation significantly influences the secondary structure. Furthermore, we investigated the structure of hPTH(1-34) in 20% trifluoroethanol solution. In addition to its helix-stabilizing effect, trifluorethanol causes the loss of tertiary hydrophobic interactions.

Role of the prosequence of guanylin

Guanylin is a guanylyl cyclase (GC)-activating peptide that is mainly secreted as the corresponding prohormone of 94 amino acid residues. In this study, we show that the originally isolated 15-residue guanylin, representing the COOH-terminal part of the prohormone, is released from the prohormone by cleavage of an Asp-Pro amide bond under conditions applied during the isolation procedures. Thus, the 15-residue guanylin is probably a non-native, chemically induced GC-activating peptide. This guanylin molecule contains two disulfide bonds that are absolutely necessary for receptor activation. We demonstrate that the folding of the reduced 15-residue guanylin results almost completely in the formation of the two inactive disulfide isomers. In contrast, the reduced form of proguanylin containing the entire prosequence folds to a product with the native cysteine connectivity. Because proguanylin lacking the 31 NH2-terminal residues of the prosequence folds only to a minor extent to guanylin with the native disulfide bonds, it is evident that this NH2-terminal region contributes significantly to the correct disulfide-coupled folding. Structural studies using CD and NMR spectroscopy show that native proguanylin contains a considerable amount of alpha-helical and, to a lesser extent, beta-sheet structural elements. In addition, a close proximity of the NH2- and the COOH-terminal regions was found by NOESY. It appears that this interaction is important for the constitution of the correct conformation and provides an explanation of the minor guanylyl cyclase activity of proguanylin by shielding the bioactive COOH-terminal domain from the receptor.

The structure of human parathyroid hormone-related protein(1-34) in near-physiological solution

Parathyroid hormone-related protein plays a major role in the pathogenesis of humoral hypercalcemia of malignancy. Under normal physiological conditions, parathyroid hormone-related protein is produced in a wide variety of tissues and acts in an autocrine or paracrine fashion. Parathyroid hormone-related protein and parathyroid hormone bind to and activate the same G-protein-coupled receptor. Here we present the structure of the biologically active NH2-terminal domain of human parathyroid hormone-related protein(1-34) in near-physiological solution in the absence of crowding reagents as determined by two-dimensional proton magnetic resonance spectroscopy. An improved strategy for structure calculation revealed the presence of two helices, His-5-Leu-8 and Gln-16-Leu-27, connected by a flexible linker. The parathyroid hormone-related protein(1-34) structure and the structure of human parathyroid hormone(1-37) as well as human parathyroid hormone(1-34) are highly similar, except for the well defined turn, His-14-Ser-17, present in parathyroid hormone. Thus, the similarity of the binding affinities of parathyroid hormone and parathyroid hormone-related protein to their common receptor may be based on their structural similarity.

Carboxy-terminal extension stabilizes the topological stereoisomers of guanylin

The peptide hormone guanylin constitutes two topological stereoisomers, which are connected through an equilibrium of interconversion. To investigate the importance of amino acid residues in the central region between the inner cysteines and at the carboxy terminus for this isomerism, synthetic derivatives of guanylin were compared by HPLC, 2D1H NMR spectroscopy and by their guanylyl cyclase-C (GC-C)-activating potency. An increase in the central sterical bulk by introduction of diiodo-Tyr9 had virtually no effect on the isomerization kinetics. Compared to guanylin, carboxy-terminal amidation did not affect the equilibrium between the two isoforms either. In contrast, two significantly stabilized isomers were obtained by extending the carboxy terminus of guanylin with one additional leucine resembling the characteristic of human uroguanylin isomers. This effect was intensified by a further Lys-Lys extension, thus revealing that the conformational exchange between the guanylin isomers is dependent on the extent of the sterical hindrance in the carboxy-terminal region of this peptide. Demonstrated by 2D NMR spectroscopy, the separated isomers of the carboxy-terminally extended derivatives of guanylin exhibit unambiguously closely related structures as found originally for guanylin isomers, which are only detectable as a mixture. Because only one of the stabilized guanylin isomers activates guanylyl cyclase-C, the three-dimensional structure of the GC-C-activating guanylin isomer is now defined. The stabilized isoforms of guanylin described in this study represent suitable tools for the separate functional investigation of the GC-C-agonistic isomer of guanylin as well as of its isomeric counterpart.

One peptide, two topologies: structure and interconversion dynamics of human uroguanylin isomers

The peptide hormone uroguanylin stimulates chloride secretion via activation of intestinal guanylyl cyclase C (GC-C). It is characterized by two disulfide bonds in a 1-3/2-4 pattern that causes the existence of two topological stereoisomers of which only one induces intracellular cGMP elevation. To obtain an unambiguous structure-function relationship of the isomers, we determined the solution structure of the separated uroguanylin isoforms using NMR spectroscopy. Both isomers adopt well-defined structures that correspond to those of the isomers of the related peptide guanylin. Furthermore, the structure of the GC-C-activating uroguanylin isomer A closely resembles the structure of the agonistic Escherichia coli heat-stable enterotoxin. Compared with guanylin isomers, the conformational interconversion of uroguanylin isomers is retarded significantly. As judged from chromatography and NMR spectroscopy, both uroguanylin isoforms are stable at low temperatures, but are subject to a slow pH-dependent mutual isomerization at 37 degrees C with an equilibrium isomer ratio of approximately 1:1. The conformational exchange is most likely under the sterical control of the carboxy-terminal leucine. These results imply that GC-C is activated by ligands exhibiting the molecular framework corresponding to the structure of uroguanylin isomer A.

Structure-activity relation of NH2-terminal human parathyroid hormone fragments

Human parathyroid hormone (hPTH) is involved in the regulation of the calcium level in blood. This hormone function is located in the NH2-terminal 34 amino acids of the 84-amino acid peptide hormone and is transduced via the adenylate cyclase and the phosphatidylinositol signaling pathways. It is well known that truncation of the two NH2-terminal amino acids of the hormone leads to complete loss of in vivo normocalcemic function. To correlate loss of calcium level regulatory activity after stepwise NH2-terminal truncation and solution structure, we studied the conformations of fragments hPTH-(2-37), hPTH-(3-37), and hPTH-(4-37) in comparison to hPTH-(1-37) in aqueous buffer solution under near physiological conditions by circular dichroism spectroscopy, two-dimensional nuclear magnetic resonance spectroscopy, and restrained molecular dynamics calculations. All peptides show helical structures and hydrophobic interactions between Leu-15 and Trp-23 that lead to a defined loop region from His-14 to Ser-17. A COOH-terminal helix from Met-18 to at least Leu-28 was found for all peptides. The helical structure in the NH2-terminal part of the peptides was lost in parallel with the NH2-terminal truncation and can be correlated with the loss of calcium regulatory activity.

Synthesis, biological activity and isomerism of guanylate cyclase C-activating peptides guanylin and uroguanylin

Recently, the peptides guanylin and uroguanylin were identified as endogenous ligands of the membrane-bound guanylate cyclase C (GC-C) that is mainly expressed in the intestinal epithelium. In the present study, bioactive guanylin and uroguanylin have been prepared by solid-phase methodology using Fmoc/HBTU chemistry. The two disulfide bonds with relative 1/3 and 2/4 connectivity have been introduced selectively by air oxidation of thiol groups and iodine treatment of Cys(Acm) residues. Using this strategy, several sequential derivatives were prepared. Temperature-dependent HPLC characterization of the bioactive products revealed that guanylin-related peptides exist as a mixture of two compounds. The isoforms are interconverted within approximately 90 min, which prevents their separate characterization. This effect was not detected for uroguanylin-like peptides. Synthetic peptides were tested for their potential to activate GC-C in cultured human colon carcinoma cells (T84), known to express high levels of GC-C. The results obtained show that both disulfide bonds are necessary for GC-C activation. The presence of the amino-terminally neighboring residues of Cys104 for guanylin and Cys100 for uroguanylin has been found to be essential for GC-C stimulation. Unexpectedly, a hybrid peptide obtained from substitution of the central tripeptide AYA of guanylin by the tripeptide VNV of uroguanylin was not bioactive.

Structure of human parathyroid hormone 1-37 in solution

Human parathyroid hormone (hPTH), amino acids Ser1 to Leu37, is biologically active with respect to both receptor binding and activation of adenylate cyclase to influence the serum calcium concentration. It induces DNA synthesis via an unknown signal pathway. We investigated the structure of hPTH(1-37) in H2O/buffer solution under near physiological conditions, that is pH 6.0 and 270 mM salt, by circular dichroism, ultracentrifugation, nuclear magnetic resonance spectroscopy, and molecular dynamics calculations. Complete sequence specific assignments of all 1H resonances were performed by using 1H two-dimensional NMR measurements (double quantum-filtered correlated spectroscopy, nuclear Overhauser effect spectroscopy (NOESY), and total correlation spectroscopy with suppression of NOESY-type cross-peaks spectra). hPTH(1-37) obtained helical structure and showed hydrophobic interactions defining a tertiary structure. The NH2-terminal four amino acids of hPTH(1-37) did not show a stable conformation. Evidence for an alpha-helical region between Ile5 and Asn10 was found. This region was followed by a flexible link (Gly12, Lys13) and a well defined turn region, His14 to Ser17. The latter was stabilized by hydrophobic interactions between Trp23 and Leu15. Ser17 through at least Leu28 formed an alpha-helix. Arg20 and Lys27 were involved in the core built by His14 to Ser17. Unrestrained molecular dynamics simulations indicated that the structure was stable on the 200 ps time scale.